This invention relates to a new and improved turn off circuit for a multiphase motor, and more specifically, to a circuit adapted to sequentially turn off positively and negatively biased SCRs, and to a control system therefor. The invention is suitable for use in diverse applications such as AC powered vehicles, stationary motors etc., and the DC energy may be supplied by storage batteries, fuel cells, solar cells, thermoelectric cells, etc. Variable speed motors of multiphase configuration such as squirrel cage, wound rotor, synchronous, linear or commutatorless DC motors and the like may utilize the turn off circuit of this invention.
A wide variety of turn off circuits are known, and some are discussed in the text, "Principles of Inverter Circuits," by B. D. Bedford and R. G. Hoft, 1964, by John Wiley & Sons, Inc. Some problems of turn off and commutation circuits include the large amount of trapped circulating current and energy which impairs performance, counter rotating harmonics, etc. It is also important to be able to feed back energy to the DC source without undue energy losses and without adversely affecting the circuit components. Furthermore, a linear power output to the motor should be provided both at low and high speeds; "cogging" at low speed should be avoided.
Overall, highly uniform turn off pulses are necessary in the case of battery powered vehicles since they are operated under a wide variety of conditions; hence, SCRs are frequently used since they have the capability for high speed switching at relatively high loads. However, because of their turn off characteristics, special means must be taken to ensure they are properly turned off without unduly complicating the circuitry or impairing their performance.